Composite ceramic multilayer substrate, heat generating element-mounting module, and method of producing composite ceramic multilayer substrate

A method of producing a composite ceramic multilayer substrate, comprising:

• a step of forming a glass ceramic insulating layer on one main surface of a highly thermally conductive ceramic insulating layer made of a ceramic material having a higher thermal conductivity than a glass ceramic contained in the glass ceramic insulating layer or both main surfaces of the highly thermally conductive ceramic insulating layer directly and/or with a wiring layer interposed therebetween;
• and
• a step of forming a heat generating element-mounting wiring line directly on a main surface of the highly thermally conductive ceramic insulating layer;
• wherein
• when the composite ceramic multilayer substrate is viewed in a direction perpendicular or substantially perpendicular to a main surface of the composite ceramic multilayer substrate, at least one heat generating element-mounting portion is formed which is surrounded by the glass ceramic insulating layer and at which the heat generating element-mounting wiring line formed directly on the main surface of the highly thermally conductive ceramic insulating layer is exposed.